Intestinal absorption and tissue distribution of carotenoids

Abstract

Human metabolism of carotenoids is of interest not only because of the provitamin A function of certain carotenoids, but also because these compounds have been associated with reducing risks of certain cancers and chronic diseases. Full understanding of carotenoid metabolism is complicated by a number of factors: variations in physiochemical properties among carotenoids; altered carotenoid utilization as a result of the normal vicissitudes of lipid absorption and transport; divergence in metabolic fate within the intestinal enterocyte (especially carotenoid cleavage to retinoids); differences in packaging and transport in lipoproteins; dissimilarity in tissue uptake of specific carotenoids; and the possible isomerization of carotenoids within tissues. Hampering research progress is the lack of animal models that perfectly mimic human carotenoid metabolism and the limited number of carotenoids approved for human consumption in a pure form. It is clear, however, that diet affects carotenoid absorption. The efficient digestion and absorption of dietary fat, as well as the presence of bile salt micelles, is essential for carotenoid absorption. Competition among carotenoids and other dietary components for absorption, transport, and uptake by tissues is documented but needs further research. From limited data it seems that the more polar carotenoids (xanthophylls) may be absorbed more efficiently than are carotenes (hydrocarbon carotenoids). The mechanism in humans for the phenomenon of “nonresponders” to oral doses of β- carotene requires further study to determine whether it is an effect of inefficient absorption or of intestinal cleavage of β- carotene. The delayed maxima in plasma carotenoid concentrations after ingestion, relative to plasma triacylglycerol, are explainable as secondary secretion of carotenoids transferred to other lipoproteins after chylomicron clearance. The site and degree of isomerization of carotenoids after consumption has not yet been defined. In this article the absorption, transport, and tissue uptake of carotenoids will be reviewed and possible differences among carotenoids and factors influencing their utilization will be discussed.